Methods and apparatuses for automatically bringing a lens into a focusing position with respect to an image in a field of view are well known in the art. In the prior art, such methods and apparatuses have employed active means, for example, sonar or infrared transmission and reception, to determine the distance of the lens to the image and to determine the best focal position therefrom.
A passive distance determination method and apparatus is disclosed in U.S. Pat. No. 5,003,165. In that patent, however, a quantile calculation which is a histogram of the intensity of light at each of the pixels in an image is determined. Thus, the calculation of the quantile for an image depends upon the content of the image. Furthermore, the method and apparatus disclosed therein, performs the function of selecting the best focal position after a plurality of images are taken of the same field of view and the focusing means is moved in response to the calculation of the histogram from each of the different positions of the focusing means.
U.S. Pat. No. 4,338,024 discloses a flow cell wherein objects, such as biological cells, are moved in a fluid past a stationary field of view. In the application of imaging technology to a flow cell where the image is captured only once, the teachings of U.S. Pat. No. 5,003,165 cannot be applied to select the best focal position since only one image of the field of view is obtained.